So far this year 1,118 cases and 41 deaths have been reported, representing "the highest numbers of West Nile virus disease cases reported to CDC through the third week in August since West Nile virus was first detected in the United States in 1999," Centers for Disease Control and Prevention vector-borne diseases Director Dr. Lyle Petersen told reporters in a phone news conference. "In comparison, one month ago, there were only 25 people with West Nile virus disease reported to the CDC."
"People around the country are understandably very concerned about the outbreak -- especially in hard-hit areas like Texas, where almost half of the cases have been reported," he said, adding the number of reported cases is "trending upward in most states, including Texas."
Doctors are not clear about why this is turning into the worst year nationally for the virus, Petersen said. Hot weather is known to increase transmission, he said, and researchers speculate the mild winter, combined with an early spring and very hot summer, made worse by a severe drought, produced the right conditions for massive mosquito breeding and the spread of the virus.
Mosquitoes catch the virus when they bite infected birds. They then pass it on to people.
The infection has been found in mosquitoes or birds in 47 states, with about 75 percent of cases in Texas, Mississippi, Louisiana, South Dakota and Oklahoma, Petersen said. Texas alone recorded 25 West Nile deaths so far, said Texas state Health Commissioner Dr. David Lakey, who was also on the call.
The only states not reporting West Nile activity are Alaska, Hawaii and Vermont.
About 80 percent of West Nile virus infections in people present no symptoms. But nearly 20 percent of infected people develop a fever, headache, body ache, vomiting, swollen lymph glands or a skin rash, which officials refer to as "West Nile fever."
About 1 in every 150 infected people can develop severe illnesses, including high fever, convulsions, vision loss, paralysis, coma and permanent or fatal neurological effects including encephalitis or meningitis -- acute inflammation of the brain or of the protective membranes covering the brain and spinal cord.
It takes two to three weeks for symptoms to develop, so the agency said it expects the number of reported cases will increase through the end of September.
In 2003, the previous record year, it reported 264 deaths.
No vaccine or drug specifically targets the virus, so health authorities advise people to take precautions to avoid getting bitten.
More than 30,000 people in the United States have come down with the West Nile virus since 1999, CDC figures indicate.